Solid state ignition system



United States Patent Inventor Ralph P. Walker 1125 Evans SL, Oshkosh, Wis. 5490! 781,186

Dec. 4, 1968 Dec. 29, 1970 Appl. No. Filed Patented SOLID STATE IGNITION SYSTEM 10 Claims, 1 Drawing Fig.

US. Cl 123/148, 315/209 Int. Cl F02p 3/06 Field of Search l23/l46.5A,

148DC, 148E; 315/209, 209T, 214

[56] References Cited UNITED STATES PATENTS 3331.986 7/l967 Hardin et al. 3 l 5/209X 3,356,896 l2/l967 Shano 315/209 3,370,190 2/ l 968 Neapolitakis 310/70 3,390,668 7/1968 l-lufton l23/l48E 3,390,669 7/1968 l-lufton 123/ 148E Primary Examiner-Laurence M. Goodridge Attorneylra Milton Jones ABSTRACT: A condenser is charged from the rectified output of an oscillating circuit and is discharged through the primary of a conventional ignition coil by triggering of an SCR. SCR triggering is timed by means of a permanent magnet cooperating with a conventional breaker cam to induce AC in a detector coil that is connected with the SCR through a two stage amplifier. Clamping circuits protect the semiconductors.

, SOLID STATE IGNITION SYSTEM This invention relates to solid state ignition systems for reciprocating internal combustion engines, and refers more particularly to a solid state ignition system that is especially well adapted to be installed on an automotive or similar engine as a replacement for its conventional ignition apparatus.

It is the general object of this invention to provide a solid state ignition system that is well adapted for retrofitting; and in furtherance of that objective it is'a more specific object of this invention to provide a solid state ignition system that utilizes a substantial amount of existing conventional ignition apparatus on an engine, and particularly its breaker cam, distributor and ignition step-up coil, without requiring anything but a minimal modification thereof. -f

Another and very important object of this invention is to provide a transistor ignition-system for automotive and similar engines that utilizes the existing breaker cam in such an engine for timing the firing ofits spark plugs, but which has no parts that are subject to mechanical wear and which requires no substantial modification of the distributor-breaker structure of the engine in effecting retrofit-conversion thereof from a con- "ventional ignition systemitofthe. solid state ignition system of this invention.

In connection with the last stated object, it is anotherand more specific object'of this invention to provide a solid state ignition system of the character described, having a condenser which is alternately charged from a power supply and discharged through an ignition step-up coil that can be identical with the step'u'p coils used in conventional ignition systems, and wherein the timing of discharge of the condenser occurs in response to the induction of a current in a small coil that is mounted adjacent to the breaker cam and in such relationship with a permanent magnet and the breaker cam that as the'breakercam rotates its lobes alternately effect thebuildup and collapse vof a flux field linked with said coil.

I 2 Electrical power for the ignition apparatus is provided by a battery 6 or other low-voltage DC source, and the purpose of the apparatus, of course, is to impress-a high voltage across each of the spark plugs 5 at the proper timein the'engine cycle for firing of the plug. In general the apparatus comprises a power supply 7 for producing a high-voltage DC by which a condenser 8 is charged. a conventional ignition step-up coil 9 through the primary of which the condenser 8 is caused to discharge at proper times and which has its secondary connected with a conventional distributor 10, a silicon controlled rectifier 12 by which the condenser 8 is discharged when firing of each plug is to occur, timing means designated generally by 13- and comprising the breaker cam 14 that is part of conventional engine ignition apparatus, and an amplifier comprising transistors 15 and 16 connected with the timing means 13 and with the SCR 12 for triggering the latter at proper times in the engine cycle. As is conventional, the distributor 10 to which the high voltage secondary of the ignition coil 9 is connected directs the output of that coil to the respective spark plugs 5 in the proper firing sequence.

The power supply 7 comprises, in general, an oscillating circuit described hereinafter, a step-up transformer l8,and a full wave rectifier network 19 that has its input terminals connected with the output terminals of the .transformer and itsoutput terminalsconnected with the condenser 8.- Preferably the power supply 7 is connected with the positive terminal of the battery 6 through a choke 21, and there is a capacitor 22 connected across the input terminals of the power supply. The choke and the capacitor 22 cooperate to prevent feedback to the'battery circuit of AC from the oscillating circuit, which might cause radio interference or affect the operation of other accessories powered from the battery circuit. As usual, the

Another object of thisinvention is to provide a solid state ignition system which is triggered by currents induced in a small coil mounted adjacent to the breaker cam, as just described,

and which provides consistenttiming of spark plug firing in relation to the position of the breaker cam, regardless of changes in the speed of the engine.

Itis also an object of this invention to provide a solid state ignition system which does not materially change its operating characteristics overa wide range of temperature variations, does not tend to cause interference with-radios and other electrical equipment that share a common electric power supply with the ignition system, and is usable-with engines of substantially widely varying speeds without drawing unduly high current at low engine speeds. i

A further specific object of this invention is to provide a solid state ignition system wherein assurance is afforded that the ratings of the semiconductors incorporated in the system will not be exceeded.

With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying draw ing. This disclosure is intendedmerely to exemplify the invention. The invention is not limited to the particular structure or method disclosed, and changes can be made therein which lie within the scope of theappended claims without departing from the invention.

The drawing illustrates one complete example of the physical embodiment of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

The single FIG. is a'circuit diagram of a solid state ignition system embodying the principles of this invention.

In the present instance'the ignition system of this invention is illustrated as applied to an engine having six cylinders, with a spark plug 5 for each cylinder, but it will be readily apparent as the description proceeds that the principles of the invention are equally well adaptable. to an engine having any other number of cylinders and spark plugs. r

derstood that a conventional ignition switch- 24 controls the connection of the ignition apparatus with the battery system.

The oscillating circuit of the power supply 7 comprises a pair of transistors 25 and 26, a pair of condensers 27 and 28, a pair of resistors 29 and 30, and a pair of diodes 31 and 32. The primary winding of the transformer 18 has a center-tap 34 which is connected with the positive side of the batterycircuit through the choke 21. The outer terminals 35 and 36 of the primary of transformer. 18 are respectively connected with the collectors of transistors 25-and 26, while the emitters of those transistors are grounded as at 38. Hence current flows in one direction, through one half of the transformer primary, when transistor 25 is conducting, and in' the opposite direction, through the other half .of said primary, when transistor 26 is conducting. Y

The base-of each of the transistors 25 and 26 is connected with the collector of the other-through a resistor and a capacitor that are in parallel with. one another; and specifically the base of transistor 25 is connected with the collector of transistor 26 through capacitor 27 in parallel with resistor 29, while the base of transistor 26is connected with the collector of transistor 25 through capacitor 28 in parallel with resistor 30.

The operation of the oscillator circuit will be apparent to those skilled in the art. Briefly, when transistor 25 begins to conduct, the potential at its collector drops, and this drop is I .sistor 30, and the potential at the base of transistor 26 begins to rise and it begins toconduct, starting a reverse cycle. As the transistors are thus alternately cut off and driven to saturation,

an alternating current is induced in the secondary of transformer 18. The values-of the constants in the oscillating circuit are so chosen that this alternating current has a relatively high frequency, as on the order of 50,000 c.p.s.

Transistors 25 and 26 are respectively protected by silicon rectifier diodes 31 and 32, each of which is connected between the emitter of its transistor and ground. Each diode becomes conductive when the back voltage across the baseemitter circuit of its transistor exceeds a predetermined value (e.g.,.7 v.), preventing that value from being exceeded and thereby protecting the transistor against burnout.

The AC at the output terminals of step-up transformer 18, which has a potential on the order of 200 v., is rectified by the full wave rectifier network 19 with which the secondary of transformer 18 is connected. A capacitor 39 is preferably connected across the output terminals of the rectifier network to provide a filter that smooths out the pulses of current from the rectifier network and prevents high voltage spikes that might exceed the maximum voltage rating of SCR 12.

The condenser 8, in series with the primary winding of the step-up coil 9, is connected across the terminals of the rectifier network 19. Specifically, the positive terminal of the rectifier network has a common connection, as at 40, with the positive terminals of capacitors 8 and 39 and with the anode of the vSCR 12. Hence when the SCR is triggered, it effects a rapid discharge of condenser 8 that causes a surge of current through the primary of the ignition coil 9.

The triggering of the SCR to render it conductive is effected in timed relation to operation of the engine by the timing means 13, acting through the amplifier comprising transistors and 16. The timing means 13 comprises the breaker cam 14, having a lobe for each spark plug, by which breaker points are mechanically actuated in a conventional ignition apparatus. In the present case, however, the breaker points are replaced by a permanent magnet 41 and a detector coil 42, both of which are small enough to be located within the breaker housing without requiring remodeling or modification thereof.

The permanent magnet 41, which can be of the ceramic type, is axially short and has its magnetic axis substantially radial to the axis of the cam. It is mounted on an arcuate magnetically permeable core member 43 that overlies the radially outer face of the magnet and extends partway around the cam, inside the breaker housing, terminating in a radially inwardly projecting pole portion 44 that provides a core for the detector coil 42. A narrow pole shoe 4S overlies the radially inner face of the magnet and projects toward the cam to cooperate with the pole portion 44 in defining a flux path through the magnet. The pole shoe 45 and pole portion 44 are circumferentially spaced apart by a distance such that lobes of the cam regularly align with both of them as the cam rotates; and because they are substantially narrow they produce a relatively intense flux concentration when cam lobes are aligned with them, which concentration diminishes rapidly as the cam lobes move out of alignment with them upon continued rotation of the cam.

It will be apparent, therefore, that rotation of the cam 14 induces in the detector coil 42 an alternating current having a positive peak and a negative peak for each alignment of cam lobes with the pole members 44 and 45. The peak values of this alternating current, however, will vary rather considerably with different engine speeds; that is the AC will have a very low peak value when the engine is idling but will have a relatively high peak value when the engine is operating at high speed. For reasons which will appear hereinafter, the amplifier comprising transistors 15 and 16 triggers the SCR 12 at the lowest as well as the highest engine speeds and is not injured by the high voltages induced in the detector coil at high engine speeds. In addition, triggering of the SCR always occurs in a predetermined position of rotation of the cam 14, and such timing is unaffected by engine speed so that the ignition timing apparatus of the engine need not be modified or substantially readjusted to compensate for the characteristics of the ignition apparatus of this invention.

The detector coil 42 is connected in a series circuit comprising a resistor 46 and the base-emitter circuit of the first amplifier transistor 15, the resistor 46 having one terminal connected to the positive terminal of the battery and its other terminal connected with one end of the detector coil. the other end of the coil being connected with the base of transistor 15, and the emitter of transistor 15 being grounded.

A silicon diode 47 is connected between ground and the junction of resistor 46 with the detector coil, in the direction to conduct to ground. Another silicon diode 48 is connected between ground and the junction of the detector coil with the base of transistor 15, in the direction to conduct from ground.

The series circuit comprising the battery 6, the resistor 46 and the detector coil 42 provides for bias on the first amplifier transistor 15, in cooperation with diode 47 which provides for temperature compensation of the bias voltage. The bias circuit tends to maintain a value of bias voltage such that when no current is induced in the detector coil 42, the transistor 15 is either just on the verge of cutoff or just on the verge of conduction. Thus any positive current induced in the detector coil 42 will render the transistor 15 definitely conductive, and even a very small current through the detector coil will cause transistor 15 to go to saturation.

The diode 47 conducts continuously to an extent dependent upon its temperature, and it is selected to match the temperature response characteristics of transistor 15, to the extent that the performance characteristics of said transistor remain constant through the range of temperatures from about 5 5C to about +1 25C thus insuring that the ignition apparatus will operate satisfactorily under any temperature conditions to which it might be subjected.

To prevent the first amplifier transistor from being subjected to an excessive back voltage, by which it might be burned out, the diode 48 conducts when the negative potential at the base of transistor 15 exceeds a predetermined value, which however varies with temperature and is in the neighborhood of 3 to The value of resistor 46 is so chosen, with respect to the conductivity of diode 47 and the internal resistance of detector coil 42, that the maximum current rating of the baseemitter circuit of transistor 15 is not exceeded, but sufficient current is permitted to flow in said base-emitter circuit to cause transistor 15 to go to saturation.

The internal resistance of detector coil 42 is high in relation to the resistance of the base-emitter circuit of transistor 15. This high resistance of coil 42 accommodates the wide variations in induced voltage across it with different engine speeds, inasmuch as the'higher voltages are mostly dissipated across the coil resistance so that the maximum positive voltage rating of transistor 15 is not exceeded.

The transistor 15 must have high gain characteristics so that it will reach saturation conduction even when very small currents are induced in the detector coil 42. Preferably its gain factor is on the order of 500 to 1,000, so that it requires only one or two microamperes of current through its base-emitter circuit to reach saturation. To meet these requirements the transistor 15 can be either a 2N5088 or a 2N5089.

The one to two microampere level of base-emitter current through first amplifier transistor 15 is attained so near the beginning of each AC cycle induced in the detector coil 42 that the sine wave form of the induced current in the coil is in effect converted to a square wave form in the current through the collector-emitter circuit of transistor 15, and the SCR IS always triggered at substantially the same points in the cycle of rotation of cam 14, regardless of variations in engine speed and resultant variations in the amplitude of the AC wave induced in the coil 42.

The collector-emitter circuit of the first amplifier transistor 15, in series with a resistor 49, is connected across the terminals of the battery or voltage source 6, and the collectoremitter circuit of transistor 16, in series with a resistor 50, is similarly connected across the terminals of the DC power supply. The base of the second amplifier transistor 16 is connected with the junction between the resistor 49 and the collector of transistor 15, and transistor 16 thus provides a second amplifier stage that is direct coupled to the first amplifier stage provided by transistor 15. Consistently with its function as a second amplifier stage, the transistor 16 is selected to pass a larger current than transistor 15,

The collector-to-emitter saturation voltage of transistor must be less than the base-to-emitter cutoff voltage of transistor 16, so that whenever the first amplifier transistor 15 is conducting, the second amplifier transistor 16 will be cut off.

I The resistor'49 that is in series with the collector-emitter circuit of first amplifier transistor 15 is of such value as to provide the proper level of collector current through said transistor 15, so that its maximum current rating will not be exceeded. The resistance value of resistor 49 must also be chosen with regard to the base circuit of the second amplifier transistor 16, and must be such that transistor 16. will attain saturation conduction when transistor 15 is cut off, but that the maximum current rating of transistor 16 will not be exceeded.

The second amplifier stage comprising thetransistor 16 is coupled to the SCR 12 by means of a capacitor 51 that has one of its terminals connected with the junction between the collector of transistor 16 and resistor 50 and has its other terminal connected with the cathode of the SCR l2. Whenever transistor 16 is not conducting, the capacitor 51 is charged through the resistor 50. When transistor 16 conducts, the capacitor 51 is discharged through it, triggering the SCR 12. The value of resistor 50 is so chosen that it will allow sufficient current flow through the collector-emitter circuit of transistor 16 as to insure triggering of SCR 12 at any temperature within its expectable operating range, but without exceeding the maximum rating of transistor 16.

Note that negative triggering is used on the SCR 12, its gate being grounded through a resistor 52 which limits the current through the gate circuit to prevent the characteristics of the SCR from being exceeded. To further protect the SCR, a silicon diode 53 is connected between its cathode and ground.

. The diode 53, which conducts when the SCR is conductive, di-

vides with the SCR any back voltage across the SCR circuit.

When the SCR l2is triggered, it effectively grounds the condenser 8, which thereupon forces current through the primary of the step-up coil 9. Discharge of condenser 8 through the SCR concomitantly charges coupling capacitor 51.

The circuit comprising the primary of step-up coil 9 and the capacitor 8 is of course a resonant one, and as current flows through the SCR due to discharge of thecapacitor 8, a reverse current is induced in that resonant circuit. When the value of that induced current exceeds the discharge-current through the SCR, the SCR is cut off. The SCR is thus self-commutating, and once cut off it thereafter remains nonconductive until it is again triggered by an impulse from the detector circuit comprising the coil, 42. During the time that the SCR is nonconductive, the condenser 8 is recharged from the power supply 7.

Preferably a resistor 54 is connected across the condenser 8 to cooperate with capacitor 39 in preventing voltage peaks in the output of the power circuit by providing a small load thereon, and to define the resonant circuit comprising capacitor 8 and coil 9.

From the foregoing description taken with the accompanying drawings it will be apparent that this invention provides a reliable solid state ignition system that is well adapted for retrofitting in engines originallyequipped with conventional that might be impressed ignition systems, and that it has an indefinitely long useful life 1 because it has no parts that are subjected tomechanical wear and is capable of dependable operation through a wide range of engine speeds and temperatures.

lclaim:

1. In an ignition system for firing the spark plugs of a reciprocating internal combustion engine having an ignition timing cam which rotates in timed relation to operation of the engine and which has a lobe for each engine cylinder:

A. an ignition step-up coil having 1. a primary winding and 2. a secondary winding connectable with the spark plugs;

B. a capacitor connected with the primary winding of the step-up coil and arranged to be discharged therethrough;

C. means for charging the capacitor;

D. an SCR connected with the capacitor for discharging the same when the SCR is rendered conductive;

E. timing means comprising 1. a permanent magnet and 2. a detector coil, both fixed adjacent to the cam and in such operative relationship thereto that as the cam rotates, the lobes thereof cause the flux linked with the detector coil to cyclically vary in value. to induce currents in the detector coil that vary in timed relation to the engine cycle;

F. amplifier means having input terminals connected with a source of low-voltage DC through the detector coil and having output terminals connected with the SCR, for rendering the SCR conductive whenever current is in duced in the detector coil, said amplifier means comprising a transistor having its base connected with the detector coil; and

G. a diode connected with said DC source in shunt across the circuit comprising the detector coil and said transistor and having temperatureresponse characteristics substantially matching those of said transistor, said diode being arranged to conduct at all times so as to cooperate with the DC source in normally maintaining the biasing'voltage on said transistor at a value near cutoff, thus rendering the amplifier means responsive to small changes in voltage across the detector coil.

2. The ignition system of claim 1, further characterized by said means for charging the capacitor comprising:

I A. astable circuit means comprising a pair of triodes having resistive-capacitive feedback coupling connectable with said source of direct current to be steadily energized therefrom, for producing an alternating current;

B. a step-up transformer having its input terminals connected with said astable circuit means; and

C. a full-wave rectifier network connected between the output terminals of the step-up transformer and the capacitor.

3. The ignition system of claim 1 characterized by said amplifier means further comprising:

A. a second transistor 1. having its base circuit directly coupled with the collector-emitter circuit of the first mentioned transistor, and

2. having its collector-emitter circuit capacitance coupled with the cathode of the SCR.

4. In an ignition system connectable with a low-voltage DC source and by which firing of the spark plugs of a reciprocating internal combustion engine is effected in timed relation to rotation of a timing cam which is driven by the engine and which has a lobe for each engine cylinder:

A. DC power supply means having input terminals connectable with the low-voltage DC source and output terminals across which a high-voltage DC can be maintained;

B. a power capacitor having one of its terminals connected with the positive output terminal of the DC power supply means; i

C. an ignition step-upcoil having a primary with one terminal connected with the negative output terminal of the DC power supply means and its other terminal connected with the other terminal of the power capacitor, so that discharge of the power capacitor draws current through said primary;

D. an SCR having its cathode connected with the negative terminal of the DC power supply means, its gate connected with the same negative terminal through re sistance means, and its anode connected with said one terminal of the power capacitor, so that the power capacitor is discharged when the SCR conducts;

E. timing means comprising:

i. a permanent magnet and 2. a detector coil, both fixed adjacent to the cam and in such operative relationship thereto that as the cam rotates, the lobes thereof cause the flux linked with the coil to cyclically vary in value, thus inducing currents in the detector coil that vary in timed relation to the engine cycle;

F. a transistor having its collector-emitter circuit connected in a series circuit with a resistor across the low-voltage DC source;

G. means connecting the base of said transistor with the detector coil so that said transistor is rendered conductive in response to current in the detector coil of a predetermined value; and

H. a coupling capacitor having one of its terminals connected with the cathode of the SC R and its other terminal connected in the last mentioned series circuit, to be charged when the transistor is cut off and to be discharged when the capacitor conducts and thereby render the SCR conductive.

5. The ignition system of claim 4, wherein said means connecting the base of said transistor with the detector coil comprises:

A. a second transistor 1. having its collector emitter circuit connected across said low-voltage DC source and coupled with the base circuit of the first mentioned transistor, and

2. having its base circuit connected across said low-voltage DC source and in series with the detector coil; and

B. a diode shunted across the base circuit of said second transistor and having temperature response characteristics substantially matching those of said second transistor, said diode being arranged to conduct at all times so as to cooperate with the detector coil in maintaining the biasing voltage on said second transistor at a value near cutoff.

6. in an ignition system for a reciprocating internal combustion engine, of the type wherein firing of spark plugs is effected in consequence of discharge of a condenser through a triggered electronic switching device under the control of an engine driven timing cam having a lobe for each spark plug:

A. means cooperating with the timing cam to produce electrical impulses in timed relation to rotation of the cam, said means comprising 1. a permanent magnet and 2. a detector coil, both fixed adjacent to the cam and so arranged that the flux field of the permanent magnet is linked with the detector coil through the cam;

B. a transistor having 1. its base connected with the detector coil in a series circuit across which a DC of low voltage is normally impressed, and

2. its collector and emitter connected in a circuit through which a relatively high current can flow,

3. said transistor being selected from the class comprising 2N5088 and 2N5089;

C. a first silicon diode, connected with the end of the detector coil remote from the base of the transistor and shunted across said series circuit, said first diode having temperature characteristics substantially matching those of said transistor and being arranged to pass current at all times so as to maintain the biasing current on said transistor at a value near cut off;

D. a second silicon diode, connected with the other end of the detector coil and shunted across the transistor, said second diode being arranged to bypass around the base of the transistor negative voltages in excess of a predeterruined value; and

E. means coupling said circuit comprising the collector and emitter of the transistor with the triggered electronic switching device.

7. The ignition system of claim 1, further characterized by:

A. said amplifier comprising the detector coil and the baseemitter circuit of the transistor connected in series across the low-voltage DC source; and

B. a second diode shunted across the base-emitter circuit of the transistor and arranged to conduct when more than a predetermined back voltage appears across the baseemitter circuit of the transistor.

8. In a semiconductor ignition system for a reciprocating engine having a cam which is rotatably driven by the engine and which has a lobe for each cylinder of the engine. means for producing electrical timing pulses by which discharges across the spark plugs of the engine at proper times in the engine cycle are controlled, said means comprising:

A. a permanent magnet having a pair of faces that define opposite magnetic poles;

B. a permeableshoe member overlying one face of the magnet and having a narrow portion projecting radially toward the cam to lie closely adjacent to the cam lobes and define a pole surface having small circumferential extent;

C. a permeable core member overlying the other face of the magnet and extending partway around the cam;

D. a core portion on said core member projecting radially therefrom toward the cam and defining a second pole surface of small circumferential extent which is so spaced circumferentially from the first mentioned pole surface that said pole faces are simultaneously aligned with cam lobes as the cam rotates and thus cooperate with the magnet and one another in charging through the cam a flux field that has substantial variation in intensity as the lobes move into and out of alignment with said pole surfaces; and

E. a detector coil wound on said core portion to be linked with the magnetic flux field therethrough and in which currents are induced in consequence of variations in said flux fields.

9. The semiconductor ignition system of claim 8, further characterized by:

A. a transistor having its base connected with the detector coil;

B. means providing a source of DC connected with the detector coil and with the transistor and which normally maintains a biasing voltage on the transistor at a value near cutoff; and

C. means connecting the collector-emitter circuit of the transistor with a source of DC at higher voltage so that a relatively large timing pulse current flows in the collectoremitter circuit of the transistor in response to induction of a voltage across the coil that is of predetermined sign and exceeds a predetermined small value.

10. In a semiconductor ignition system for a reciprocating engine, having means providing a source of high-voltage DC, an SCR connected with said source of high-voltage DC and adapted to be triggered at appropriate times in the engine cycle to provide for flow of high-voltage current to spark plugs, an ignition timing cam which rotates in timed relation to the engine cycle and which has a lobe for each engine cylinder, and timing means comprising a magnet and a detector coil fixed adjacent to the cam in such operative relation thereto that as the cam rotates a current is induced in the coil that alternates in timed relation to the engine cycle but varies in peak magnitude with engine speed, means for utilizing the alternating current induced in the coil to trigger the SCR at substantially a predetermined time in each engine cycle irrespective of engine speed, said means comprising:

A. amplifier means 1. having its input connected with the detector coil and 2. having its output connected with the SCR, said amplifier means comprising a high gain amplifying element which is biased to a value near cutoff and which becomes saturated at a small value of current through the detector coil so that the amplifier is capable of effecting triggering of the SCR at a time very shortly after that at which the value of current through the detector 1 coil goes through zero; and B. means for limiting flow of current through said amplifying element to a value near its saturation value.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- Patent 3,550,572 Dated ngggmber 29, 1970 Inventor(s) Ralph P. Walker It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 Line 35 insert ".7vv" after to Signed and sealed this 27th day of April 1971.

(SEAL) Atte st:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

